Determination of the position of an object to be imaged, parallel to the optical axis of an optical device (z position), may be desirable for various reasons. For instance, by means of the known z position, it may be possible to position the object optimally within the focal plane of the optical device, so as to be able to produce a particularly sharp image of the object. In the case of objects which are extended perpendicularly to the optical axis, it may be desirable to determine the z position for different points of the object perpendicularly to the optical axis, in order to be able to focus the relevant image section. It may also be desirable to compile a height profile of the object by means of optical techniques.
Existing techniques, for example, allow determination of the z position by means of positioning the object at different reference positions. With the aid of the sharpness of the object at the different reference positions, it is then possible to assess whether or not the object lies in the focal plane. Often, however, it may be possible to determine a sharpness of the object only with limited accuracy. For this reason, such a previously known technique may be relatively inaccurate. Interferometric techniques for determining the z are furthermore known. Such techniques allow relatively high accuracy in the determination of the z position; the corresponding devices, however, may be relatively complex and expensive.